Ammonium nitrate composition



United States Patent 3,056,702 AIVEWGNIUM NETRATE CUMPOSITION Jack Linsk, Highland, ind, assignor to Standard ()il Company, Chicago, Ill., a corporation of Indiana No Drawing. Filed Aug. 29, 1957, Ser. No. 681,877 4 Claims. (Cl. 149-19) This invention relates to ammonium nitrate compositions suitable for use as explosives, gas generators, or in rogket propulsion.

jimmonium nitrate is a desirable component of explosives -because of its stability and cheapness. Ammonium nitrate is particularly suitable for uses wherein a continuoils evolution of high pressure gas is desired, said evolution to be over a definite period of time. Gas generators and rocket motors require such compositions. In general, ammonium nitrate compositions have the drawback of rather low burning rates. Herein burning rate is to be understood as the number of inches of composition burning in one second at a chamber pressure of 1000 p.s.i.a.; such burning rates are generally measured in 21 Crawford bomb. Thus, compositions which consist essentially of ammonium nitrate and oxidizable binder and Prussian blue combustion catalyst generally burn in the region of 0.10 to 0.13 inch per second. For many military uses such a burning rate is far too slow and the rates are increased by the addition of burning rate promoters such as carbon black. Even with the use of carbon black it is difficult to obtain burning rates of this type of com position in excess of 0.20 inch per second. For use in rocket missiles burning rates in excess of 0.20 inch per second are required.

An object of the invention is an ammonium nitrate composition of improved burning rate. A further object is an ammonium nitrate composition employing Prussian blue catalyst having a burning rate in excess of 0.20 inch per second. Other objects will become apparent in the course of the detailed description.

It has been discovered that the burning rate of an ammonium nitrate composition consisting of Prussian blue combustion catalyst, oxidizable thermoplastic binder, which binder consists essentially of polyvinyl acetate, and the remainder essentially ammonium nitrate is greatly improved and can be increased to about 0.20 inch per second by adding to the above composition a burning rate promoter selected from the class consisting of triethanolamine, triisopropanolamine, N-(hydroxyethyD-morpholine and (tetrahydroxyethyl)-ethylene diamine. In general between about 0.5 and 5 weight percent of the defined burning rate promoter is present in the composition containing said promoter.

The term ammonium nitrate as used in this specification and in the claims is intended to mean either ordinary commercial grade ammonium nitrate such as conventionally grained ammonium nitrate containing a small amount of impurities which may be coated with a small amount of moisture-resisting material such as petrolatum or paralfin; or to mean military grade ammonium nitrate or mixtures of minor amounts (usually less than of other inorganic nitrates such as sodium nitrate or potassium nitrate with the ammonium nitrate. A mixture of finely ground and coarsely ground ammonium nitrate is preferred, and the major proportion of the ammonium nitrate is finely ground. I prefer to grind the ammonium nitrate with a part or all of the catalyst component of the composition.

The composition of the invention contains ammonium nitrate as the major portion thereof. Usually the composition will contain between about 70 and about 90 weight percent of ammonium nitrate. The exact amount of ammonium nitrate will be determined by the type of oxidizable binder and the need for an oxygen balanced 3,056,702 Fatented 0st. 2, 1962 ICC composition. An oxygen balanced composition is one which theoretically burns to produce only nitrogen, water, carbon dioxide and total combustion products of the catalysts and other additives which may be present. It is to be understood that frequently compositions are deliberately not balanced with respect to oxygen demand.

The polyvinyl acetates employed in the binder material of this invention are transparent solid vinyl acetate homopolymers of relatively high molecular weight, preferably pulverized to pass through a #16 US. standard mesh screen. The materials do not have sharp melting points. The lowest molecular weight polyvinyl acetates suitable for use as base material for the binders of this invention show softening points above about 190 C. Decomposition begins at temperatures of 200 C. to 250 C. These polymers have a specific gravity of about 1.20 at 20 C.

The viscosity of the polymers in general lies within the range of from about 50 to about 900 centipoises. For purpose of this specification and the claims based thereon, the term viscosity is defined as viscosity in centipoises of a benzene solution containing 86 grams of the resin per 1000 cc. of solution determined at 20 C. with an Ostwald viscosimeter.

The ammonium nitrate composition of the invention is formed to shaped particles. For use in gas generators and rocket motors the shaped charges are of uniform configuration such as rods, tubes, stars or rods with various shaped holes cut therein such as five-pointed star. In order to form the shape the ammonium nitrate and other components such as catalyst and promoter are mixed with an oxidizable thermoplastic binder. Suflicient binder is used to permit the formation of a shape which retains its configuration at atmospheric temperatures, that is, it does not slump at high temperature or crack at a low temperature. The binder is a thermoplastic material which is a solid at atmospheric temperatures but softens to a viscosity low enough to permit ready mixing with the solid ammonium nitrate and other solid components of the composition. In general, the oxidizable thermoplastic binder is present in the composition in an amount between about 5 and 25 weight percent. The precise amount of binder is dependent upon the type of material forming the binder as Well as the requirements for the particular ammonium nitrate grain.

An essential ingredient of the binder of this composition is polyvinyl acetate which has been defined above. In general, the polyvinyl acetate will be present in the binder in an amount between about 20 and 60 weight percent. In order to form a satisfactory binder it is necessary to have present a plasticizer for the polyvinyl actate. The plasticizer itself must be an oxidizable material which reduces the softening point of the polyvinyl acetate preferably to form a binder material which is workable at temperatures on the order of C.; this temperature is usually considered as about the maximum temperature to which ammonium nitrate should be heated for safety reasons.

The plasticizer component of the binder may be any one of those materials which will function with polyvinyl acetate as a plasticizer therefor and which are in them selves oxidizable. Numerous examples of plasticizers for polyvinyl acetate are given in Plasticizers by Buttrey. Particularly desirable plasticizers are materials containing aromatic nuclei with nitro group substituents; the presence of oxygen in the compound is advantageous. The plasti- I cizer may be a single compound or it may be a mixture of two or more compounds. In many cases the plasticizing ability of the nitro-aromatic compounds is improved by the presence of substituted alcohols and acids. As the plasticizer material is not an essential feature of the invention it is not deemed necessary to spell out in great detail such materials. For ready reference there are set out below classes of compounds which have been found to be suitable plasticizers for polyvinyl acetate and use in the composition of the invention. These classes of compounds are acetins, ethylene glycol diglycolate, polyhydric alcohol maleates, hydroxy-lower-alkyl-lower alkanoates, ethylene glycol di-lower-alkanoates, nitromethyllower-alkandiol, di-loweralkanoates, the lower alkyl carbitol diglycolates, lower-alkyl alkylene diglycolates, dilower-alkyl-di-glycolates, tri-lower-alkyl citrates, acetyl tri-lower-alkyl citrates, di-lower-alkyl-phthalates, di-loweralkyl nitrophthalates, dinitrophenyllower-alkyl ether, dinitrophenyl-lower-alkylene ethers, nitrodiphenyl ethers, nitrobenzenes, di-lower alkoxy tetraglycols, lower-alkylene glycols. The terms lower-alkyl, lower-alkylene and loweralkanoate are to be understood as including from 1 to about 8 carbon atoms in the defined group.

Examples of these compounds are set out below: glycol maleate, diethylene glycol oxalate, ethylene glycol diglycolate, diethylene glycol diglycolate, monoacetin, diacetin, triacetin, hydroxyethylacetate, ethylene glycol diacetate, diethylene glycol diacetate, triethylene glycol diacetate, triethylene glycol di-Z-ethylbutyrate, polyethylene glycol di 2 ethylhexoate, nitromethylpropanediol diacetate, methylcarbitol diglycolate, di-methylallyl diglycolate, dibutyl diglycolate, tributyl citrate, tripropyl citrate, triethyl citrate, trimethyl citrate, acetyl triethyl citrate, dirnethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, ethyl glycolyl methyl phthalate, dimethyl nitrophthalate, dinitrophenyl propyl ether, dinitrophenyl allyl ether, nitrodiphenyl ethers, dinitrobenzene, nitrotoluene, dinitrotoluene, dimethoxy tetraglycol, diethoxy tetraglycol, diethylene glycol, triethylene glycol, polyethylene glycol (200), dipropylene glycol.

The ammonium nitrate composition needs a combustion catalyst to cause it to burn and in the composition of this invention the combustion catalyst is a Prussian blue. Insoluble Prussian blue is particularly suitable.

In addition to the defined catalyst, burning rate promoter and oxidizable thermoplastic binder the composition may contain additional burning rate promoters such as finely divided carbon and magnesium oxide; also stabilizers for the composition such as aromatic hydrocarbon amines may be present; also materials which improve the moldability or extrudability of composition such as surfactants broadly may be present. These other materials which are frequently introduced into ammonium nitrate compositions are to be understood as included in the term remainder essentially ammonium nitrate which appears in the claims.

The results obtainable by the use of the defined burning rate promoters of the invention in various compositions are illustrated by the following tests. Burning rate test strips of the gas-forming composition are prepared by extruding or molding the homogeneous gas-forming composition at a temperature below about 120 C. under a pressure of about 2000 p.s.i., the test strands being A" to in diameter and about 5" long. The strands are coated with lacquer grade cellulose acetate and burned in a Crawford bomb under nitrogen pressure. Four to six strands are burned at varying pressures between 600 and 1800 p.s.i. Plotting burning rate in inches per second against pressure on loglog paper gives a straight line. The slope of this straight line is defined as the exponent of the burning rate as related to pressure in the formula where B is the linear burning rate at pressure p, 18 is the linear burning rate for the composition at 1000 p.s.i., p is pressure in p.s.i. in the burning chamber and n is the pressure exponent showing dependence of the burning rate on pressure. This relationship of burning rate and pressure is discussed by R. N. Wimpress in Internal Ballistics of Solid Fuel Rockets (1950).

Gas-producing grains are prepared from the above com- 4t positions containing ammonium nitrate, combustion catalyst and binder material by molding under a pressure of about 2000 to 4000 p.s.i. The size and shape of the grains are dependent upon their intended use. Grains are provided with centrally located holes of different shapes, that is, starform, cruciform or circular. Each grain has the external surface inhibited by a coating of a material such as asphalt to restrict burning to the internal surface. Test grains subjected to tests described hereinbelow and for use in a test motor, which grains were 2.75 in diameter and about 4" in length, were provided with a starform centrally located hole. The test data obtained by burning such grains in the test motor indicate overall performance of the compositions when used in assist takeoff operations. For these operations the grains are mounted in a conventional case provided with a suitably placed igniter charge such as cannon powder, which is fired electrically. The igniter flame in turn ignites the propellant grain. The temperature of the gases produced by burning of grains may be of the order of 1500 F. to 3400" F. and the pressure or impulse produced by the hot gases will be dependent upon the grain size, diameter of the nozzle, and other factors. The gas-producing grains may be molded into disc-form, stacks of discs being used as gas-forming propellant material for missile rockets.

Test 1 In this test the oxidizable binder consisted of 40 percent commercial polyvinyl acetate, 10 percent of 2,4-dinitrodiphenyl oxide, 30 percent of dinitrophenyl propyl ether, and 20 percent of polyethyleneglycoI-di-Z-ethylhcxoate. The base composition consisted of 13 percent of this binder, 79 percent of ammonium nitrate, C.P. grade, 3 percent of insoluble Prussian blue, 3 percent of commercial carbon black beads and 1 percent of magnesium oxide. It is to be understood that in all instances percentages are given as percent by weight. The base composition burned in a Crawford bomb at 1000 p.s.i.a. at a rate of 0.179" per second and had a pressure exponent of 0.59.

A composition of the invention was made up consisting of 13% of binder, 77% of ammonium nitrate, 3% of insoluble Prussian blue, 3% of carbon black beads and 3% of triethanolamine. In the Crawford bomb at 1000 p.s.i.a. this composition burned at 0.234" per second and had a pressure exponent of 0.57.

TestZ In the grains of this test the binder had the same composition as that of Test 1. In this test the base material contained 13% of binder, 84% of ammonium nitrate and 3% of insoluble Prussian blue. This base grain burned at a rate of 0.126" per second with a pressure exponent of 0.68.

The base material was modified by the addition of carbon black beads and magnesium oxide to afford a grain consisting of binder 15%, ammonium nitrate 79%, insoluble Prussian blue 3%, carbon black beads 2%, and magnesium oxide 1%. This grain burned at a rate of 0.179 inch per second at a pressure exponent of 0.59.

A grain embodying the invention was made of 13% binder, 78% ammonium nitrate, 3% insoluble Prussian blue, 2% carbon black, 1% magnesium oxide and 3% triisopropanolamine; this grain burned at 0.220 inch per second at a pressure exponent of 0.68.

Test 3 The base material in this test consisted of 15% of the binder of Tests 1 and 2, 79% ammonium nitrate, 3% of insoluble Prussian blue, 2% of carbon black and 1% magnesium oxide. The base material had a burning rate of 0.179 inch per second. Embodiment of the composition of the invention was made consisting of 13% of binder, 77% of ammonium nitrate, 3% of Prussian blue, 3% of carbon black, 1% of magnesium oxide and 3% of (tetrahydroxyethyl)-ethylene diamine. This embodiment had a burning rate of 0.261 inch per second.

Test 4 In this test the base material of Test 3 was modified to consist of binder 13%, ammonium nitrate 77%, insoluble Prussian blue 3 carbon black 3 magnesium oxide 1% and N-(hydroxyethyl)-morpholine 3%. This composition had a burning rate of 0.264 inch per second and a pressure exponent of 0.62.

Thus having described the invention, What is claimed is:

1. An ammonium nitrate composition consisting of (a) between about 0.5 and 5 weight percent of a burning rate I promoter selected from the class consisting of triethanol nitrate, said nitrate forming the major portion of said composition.

2. The composition of claim 1 wherein said promoter is triethanolarnine.

3. The composition of claim 1 wherein said promoter is triisopropanolamine.

4. The composition of claim 1 wherein said catalyst is insoluble Prussian blue.

References Cited in the file of this patent UNITED STATES PATENTS 1,827,675 Snelling et a1 Oct. 13, 1931 2,171,379 Wahl Aug. 29, 1939 FOREIGN PATENTS 655,585 Great Britain July 25, 1951 

1. AN AMMONIUM NITRATE COMPOSITION CONSISTING OF (A) BETWEEN ABOUT 0.5 AND 5 WEIGHT PERCENT OF A BURNING RATE PROMOTER SELECTED FROM THE CLASS CONSISTING OF TRIETHANOLAMINE, TRIISOPROPANOLAMINE, N-(HYDROXYETHYL) -MORPHOLINE AND (TETRAHYDROXYETHYL)-ETHYLENE DIAMINE, (B) A PRUSSIAN BLUE COMBUSTION CATALYST, (C) AN OXIDIZABLE THERMOPLASTIC BINDER CONSISTING ESSENTIALLY OF POLYVINYL ACETATE, AND (B) THE REMAINDER ESSENTIALLY AMMONIUM NITRATE, SAID NITRATE FORMING THE MAJOR PORTION OF SAID COMPOSITION. 